I. Field of the Invention
The present invention relates to positive locking differentials for providing a positive drive to both wheels of a vehicle, while allowing differential action when required.
II. Description of the Prior Art
Many of the earliest mechanically driven vehicles were of the tri-wheel type with power and steering being provided to the single wheel, while the two remaining wheels were free to independently rotate on their axles as required when making turns. These designs were soon discarded in favor of the four-wheel type of vehicle which not only increased stability and improved appearances, but provided for a much better distribution of weight. The earlier four-wheeled vehicles had power transmitted to one wheel by means of a chain and sprocket arrangement, while the other rear wheel remained free to rotate independently on turns. The first practical installation of a bevel gear type differential permitted the four-wheel vehicle to be driven by two powered rear wheels, yet one wheel could rotate faster or slower than the other as required when turning corners or traveling on uneven surfaces; however, under certain conditions when one wheel slipped or lost traction, the other received little or no driving torque, thereby allowing the vehicle to stall. The development of modern-day power transmission units for driving axles of motor vehicles follows the same general design of operating characteristics used by their predecessors for the past 100 years. The present-day bevel gear differential remains the same except in size, material, specification and number of components. For many years attempts have been made to overcome the principal disadvantage of the conventional differential, that is, of one free spinning wheel permitting the vehicle to stall. Positive locking differentials have been devised for over-coming this problem and generally comprise a spider having fixed driving clutch teeth which are engageable with fixed teeth on a disengageable clutch. The clutch member disengages from the spider to overrun the same to accommodate a road condition or to negotiate a turn.
It can be seen that in the positive locking differential mechanism when turning a corner, the outside wheel must rotate faster than the inside wheel, otherwise serious tire scuffing would occur. When driving around a turn, the positive locking clutch driving the outside wheel is automatically disengaged, permitting the wheel to rotate freely until the turn is completed and the clutch is reengaged. When the turn is being made, there will be a series of clicking sounds resulting from the alternate disengagement and engagement of the differential clutch teeth on the outside clutch. These clicking sounds, which are quite audible on small trucks and pickups, represent a major disadvantage of the positive locking differential mechanism. This undesirable characteristic has been eliminated by means of the hold-out rings which are displaced by a key in the spider to a position between the fixed cams of the clutch members, thus preventing the clutch teeth from reengaging until the overriding cycle has been completed. Generally this consists of providing an axial groove in the teeth of the driven clutch into which the hold-out ring is snapped. The axial groove includes a slight under-cut, protruding radially inwardly at the bottom of the groove. Generally each hold-out ring is slotted and includes a shoulder protruding radially inwardly which locks into the under-cut groove of the clutch and includes a plurality of lugs protruding axially inwardly. When the clutch and hold-out ring are assembled to the spider and center cam assembly, the gaps between the ends of each hold-out ring mesh with the long spider key and the axial lugs mate with the center cam slots.
When a clutch and hold-out ring assembly is required to rotate faster to make a turn through the overriding action of one wheel, the clutch is free to ride up over the cams of the center cam, while the other clutch is held in a firm, locked engagement with the spider by the natural resistance of the slower wheel. After the wheel begins to rotate forward, the shoulder on one end of the slot and the right hold-out ring engage the spider key, locking it to the spider cam and setting its lugs ahead of the slot in the center cam. This prevents the driven cam from returning to engagement as long as it rotates faster than the spider and center cam assembly is being driven. When the overriding movement ceases and the relative speed of the spider and overriding clutch become the same, there is a slight reversal of torque so that the right hold-out ring rotates back from the spider key and its lug becomes realigned with the center cam slot, permitting the driven clutch and hold-out ring to return to full engagement with the spider center cam.
In a article, "No Spin and Tandem-Lock Differentials--Operations and Applications," by A. A. Choma, Society of Automotive Engineers No. 720904 presented at the National Commercial Vehicle Engineering and Operations meeting at Fort Wayne, Ind., on Oct. 9-12, 1972, it is indicated that automatic locking differentials are normally manufactured in three styles; namely, the standard type including individual cam indexing, the style and type including hold-out ring index control, and the silent overriding type including directional hold-out ring index control. The general operation of the standard and silent types is similar. As aforementioned, the driven clutch member disengages from the spider to override to accommodate a road condition or to negotiate a turn to the right or left in either forward or reverse directions. The standard type reengages after each clutch tooth, resulting in the aforementioned clicking or indexing sound. The silent type is equipped with hold-out ring cams which are displaced by the key in the spider to a position between the fixed cams of the clutch member, thus preventing the clutch teeth from reengaging until the overriding cycle has been completed. The silent, overriding type differential is for use in vehicles that may be particularly sensitive to the cycling action in the standard or silent type of automatic locking differentials when torque is reversed, such as over-hung engine, two-wheel tractors used in construction work. It is effective on vehicles having critical steering characteristics.
While the aforementioned positive locking differential has functioned well to eliminate the aforementioned disadvantages of noisy operation, the modified machining and additional components necessary to have the hold-out ring in each clutch on opposite sides of the spider represent a substantial cost in both labor and material. It would thus be advantageous and would represent a considerable improvement over the prior art if a positive locking differential mechanism of the type described were provided with the clutch overriding the spider teeth and being maintained in the overriding mode through a substantial degree of rotation without the necessity of the hold-out ring. In addition to the aforementioned publication of the Society of Automotive Engineers, additional prior art consists of U.S. Pat. Nos. 2,667,088; 2,667,087; 2,638,794; 2,231,968; 2,329,075; 2,830,466; 3,397,593; and 3,791,238; and the operation no-spin differential operating manuel published by Detroit Automotive, pages 1-12, 1973. These aforementioned patents and publication are relevant in that they are cited in the aforementioned copending United States patent application.